The present disclosure relates generally to systems and methods for magnetic resonance imaging (MRI) and, in particular, to systems and methods to low power simultaneous multi-slice excitation and refocusing.
MRI systems collect data in the Fourier transform representation of a scanned object and allow for non-invasive investigation of tissues with detailed contrast. Two-dimensional (2D) imaging is inherently slow since it generally involves a sequential acquisition of multiple slices that form a region of interest, wherein the total imaging time is proportional to the number of slices acquired. As a result, simultaneous multi-slice (SMS) MR imaging has gained much attention during the last several years. Its basic principle is to concurrently excite and record multiple imaging slices and subsequently use parallel imaging techniques to unfold the resulting overlapping images. This has enabled significant increase in temporal efficiency of 2D imaging acquisitions, especially for SMS echo-planar imaging (EPI), which has been proven to be a reliable method for functional and diffusion MRI.
A first step in SMS imaging is to excite a number of multiple slices at the same time. A conventional approach for creating such a multi-band (MB) excitation pattern involves summing up several single-slice radiofrequency (RF) pulses with different phase slopes (FIG. 1). The superimposed RF waveform then leads to slice excitations at multiple chosen locations. However, a major side effect of the summation is a linear growth of transmitted energy and peak power with the number of simultaneously excited slices. Such increase in energy and peak power limits full usability of MB pulses at high field strengths, particularly, for example, for spin-echo based acquisitions at 7 Tesla.
Some attempts to reduce the peak RF power of a multi-band pulse have included introducing an optimized phase term for each of the summed up RF pulses. Other strategies have implemented a time shifting approach for the RF pulse. However, both of these approaches only decrease the peak RF power and not the total energy transmission. By contrast, an alternative approach aimed to reduce both the peak RF and the total energy transmission has been to employ a variable rate selective excitation (VERSE) algorithm. However this method can be limited by pulse duration constraints and susceptibility gradients between different tissues, which can overlay weak slice selection gradients and therefore distort and shift slice profiles. Therefore, at ultra high field strength, the VERSE algorithm cannot reduce RF energy of a MB pulse sufficiently to create a high flip angle pulse with a large multi-band factor.
Recently a RF pulse type was introduced for periodic slice excitation of multiple slices, namely a power independent of number of slices (PINS) pulse, whereby a periodic slice excitation pattern is created by a constant under-sampling of a single slice RF pulse in k-space. Specifically, PINS pulses do not have a continuous RF and gradient waveform, but consist of alternating rectangular sub-pulses played out in between gradient blips (FIG. 1). This results in the excitation of periodic slice excitation ghosts, which is independent of the chosen number of slices. Therefore, in contrast to MB pulses, the RF energy of PINS pulses does not increase with a higher number of excited slices and so the energy transmission is generally lower than that of corresponding MB pulses. This distinct feature makes PINS pulses applicable to ultra-high magnetic field applications, where specific absorption rate (SAR) constraints, defined as the RF power absorbed per unit of mass of an object, can potentially limit the capability of SMS technology. As such, periodic PINS excitation has been successfully employed to enable both slice accelerated spin echo functional MRI and slice accelerated high-resolution diffusion MRI at 7 T.
However, although PINS pulses effectively enable slice acceleration at high field strengths, there are several drawbacks to this method. In particular, the composition of PINS pulses from rectangular RF sub-pulses, played out using non-constant gradient blips, prevents a fast traversal of excitation k-space. Due to physiological gradient slew rate limitations; it is not possible to speed up the gradient blips significantly, which results in a slow k-space traversal and high sensitivity to off-resonance effects, causing excitation shifts along the slice direction. One way to speed up k-space traversal is to shorten their RF sub-pulses. However, since the RF sub-pulses are only applied during a relatively small portion of the total PINS pulse duration, a large reduction in their length necessitates a large increase in the RF sub pulses' amplitudes. Yet since pulse amplitudes are restricted by RF amplifiers and by SAR constraints, this places limitations on possible reductions in RF sub-pulse durations.
Therefore, given the above shortcomings, there is a need for magnetic resonance imaging systems and methods including low power simultaneous multi-slice excitation and refocusing.